1. Field of the Invention
The present invention relates to a method for manufacturing lithium-manganese (Lixe2x80x94Mn) oxide powders which are used for positive electrode materials of Li secondary batteries, and more particularly, to a method for manufacturing Lixe2x80x94Mn oxide powders for use in positive electrode materials of Li secondary batteries, in which a battery having a high productivity, a large capacity, and a long life cycle is obtained since powders having an excellent crystallization can be produced by heat-treating for a considerably shorter time than those of conventional methods.
2. Description of Prior Art
In general, in the case that a Li secondary battery is charged, lithium (Li) existing in LiMn2O4 powders being a positive electrode material is extracted and dissolved in an electrolyte, and the dissolved lithium ions are moved to a negative electrode carbon or graphite. Meanwhile, in the case that the Li secondary battery is discharged, lithium is separated from carbon and inserted into a crystalline lattice of the LiMn2O4 powders again. In this way, since the structure in the lattice of the LiMn2O4 powders is destroyed at the time of insertion and extraction of lithium, or Mn+3 ion is dissolved into the electrolyte, a life cycle of the positive electrode is lowered. In order to prevent the life cycle of the positive electrode from being shortened, it is the most essential to manufacture powders capable of controlling a composition in chemical quantitative fashion, and having no foreign matter and an excellent crystallization.
Conventionally, for these reasons, a solid state reaction method is used in order to mass-produce powders having no foreign matter and an excellent crystallization, in which LiCO3 or LiOH and MnO2 are well mixed and maintained for 150 hours at 400xc2x0 C. or so, and then maintained again for 24 hours or more at 750xc2x0 C. This conventional solid state reaction method gives an excellent crystallization but requires a long thermal treatment time, to accordingly cause the larger particle size. Thus, it is difficult to use it at the high current condition.
A sol-gel method is known as a method of manufacturing fine oxide powders, in which polyethylene glycol, citric acid, ethylene glycol or glycolic acid is dissolved in a metal salt solution, dried and thermally treated. Using this sol-gel method, LiMn2O4 powders having an excellent property are experimentally manufactured. However, there are several problems to solve industrially. In this method, metal nitrate is generally used as a metal salt material. Since lithium nitrate (LiNO3) or manganese nitrate (Mn(NO3)2.6H2O) a main component material which is used for manufacturing the LiMn2O4 powders to be produced in the present invention well absorbs moisture in the air and reacts with the moisture, the material should be completely dried in a vacuum drier at 120xc2x0 C. or so before weighing the material, in order to control the chemical quantitative composition so that metal ions of a certain quantity exist in the material. Also, since the manganese nitrate (Mn(NO3)2.6H2O) is decomposed at too high temperature of the drier, and once it reacts with moisture to become (Mn(NO3)2.xH2O), it is not possible to see a value of x. In this case, a metal salt solution should be necessarily analyzed to measure a metal ion concentration. As a result, an accurate chemical composition cannot be easily obtained.
Meanwhile, it is usual to dry the material for a long time, e.g. 48 hours or longer, at 100xc2x0 C. or lower, in the drying process. In the case that a drying temperature becomes 100xc2x0 C. or higher in order to shorten a drying time, an abrupt decomposition reaction of organic matter remaining in powders occurs at a final thermal treatment process, and the powders are dissipated in a reaction furnace. Accordingly, since a yield is low and the reaction furnace is polluted, it is difficult to use the conventional sol-gel method industrially.
During the time when the inventors endeavor to overcome the defects found in the conventional solid state reaction method and sol-gel method, it has been found that the above conventional defects can be solved by the processes of using oxide or carbonate as a positive electrode material, drying a solution at high temperature higher than 150xc2x0 C., and putting the resulting matter into a reaction furnace in order to calcine for a short time, after treating a spontaneous combustion.
To solve the prior art problems, it is an object of the present invention to provide a method for easily synthesizing fine LiMn2O4 powders having an excellent crystallization, industrially.
To accomplish the above object of the present invention, according to the present invention, there is provided a method for manufacturing LiMn2O4 powders for use in a lithium secondary battery as a positive electrode, characterized in that oxide or carbonate is used as a positive electrode material, a solution is dried at high temperature higher than 150xc2x0 C., and the resulting matter is put into a reaction furnace in order to calcine for a short time, after treating a spontaneous combustion.
For a positive electrode material which is appropriate for the embodiment of the present invention, oxide, carbonate, hydroxide, or pure metal, at least one of which does not absorb moisture in the air, is used. For example, it is possible to use MnO2, Mn3O4, Mn2O3, MnCO3 or manganese metal as a main component in the case of manganese. Also, in the case of lithium, Li2CO3, LiOH.H2O or lithium acetate is adapted for the present invention.
Here, manganese is completely dissolved in nitric acid as a solvent in which lithium prepared so that a composition ratio with respect to manganese meets a predetermined ratio is completely dissolved, to thereby produce a metal salt solution. As a result, a chemical quantitative composition can be simply controlled. Here, an amount of the nitric acid solution used as the solvent is enough as an amount necessary to completely dissolve the manganese used. For example, in the case of MnO2 of 2M, a nitric acid solution 2 liters of 2M can be used. Here, it is preferable that a hydrogen peroxide solution is added little by little until the MnO2 is completely dissolved.
In the above, a ratio of lithium and manganese is 0.95xcx9c2.00 in 2Li/Mn. If the ratio is less than 0.95, lithium content becomes deficient, resulting in a large decrease in a battery capacity. If the ratio exceeds 2.00, an undesired crystalline phase having no electrochemical activation is produced.
After manufacturing a metal salt solution by the above process, a composition of citric acid and ethylene glycol composed as a predetermined molar ratio is dissolved in distilled water to then produce a mixture solution, and the mixture solution is mixed with the metal salt solution, by which a metal salt solution for use in manufacturing a positive electrode material is prepared.
Preferably, the molar ratio of ethylene glycol to citric acid is 0xcx9c5. If the ratio deviates from the above value, a drying time becomes long and a spontaneous combustion occurs in a drying process, to thereby cause an undesired result.
A method for manufacturing powders for a positive electrode material of a lithium secondary battery according to the present invention includes the steps of heating the obtained metal salt solution at high temperature of 150xcx9c200xc2x0 C., evaporating moisture and making the solution into the state such as a resin, sufficiently drying the moisture evaporated resulting matter, and then a spontaneous ignition and combustion is performed in the air. During the time when the resin is produced, the metal salt solution is dried in a vacuum drier at least one time at 120xcx9c200xc2x0 C. By doing so, it is preferable that a irregular combustion process is suppressed until the metal salt solution is completely dried.
The spontaneous combustion process enables a combustion of an organic matter although there is no additional external energy. Also, as an effect, a considerable amount of crystalline phase is formed by heat generated during burning of the resin, and the reaction furnace is prevented from being polluted due to an rapid reaction occurring during performing a thermal treatment. The spontaneous combustion process is performed by a general method. For example, it is sufficient if the metal salt mixture of the resin type is ignited using a lighter or match, without having any particular limitation.
The combustion product obtained via the spontaneous combustion process is input in a furnace and thermally treated for a predetermined time, to thereby produce lithium-manganese oxide powders (LiMn2O4). Since the thermal treatment process is performed at the state where powders are crystallized to a degree by the spontaneous combustion process, the thermal treatment time is shortened to a considerable degree in comparison with a conventional solid state reaction method. Thus, according to the present invention, it is possible to manufacture a lithium-manganese oxide of a single phase only with a thermal treatment of 2xcx9c24 hours performed at 600xcx9c800xc2x0 C. in the air in order to make crystallization of the combustion powders much better.
Also, the present invention can shorten a thermal treatment time, in which a heating rate is 3xc2x0 C./min or faster and a cooling rate is 0.5xcx9c10xc2x0 C. /min.
In the present invention, it is possible to substitute a part of manganese by transition metal such as Co, V, Ni, Cr, Cu, Ti, or Zn, or metal having a valence of +1, +2 or +3, such as Al, Li or Sn.